Optical filter

ABSTRACT

An optical filter includes a light-shielding conductive layer provided with a plurality of apertures on a substrate surface that selectively transmits light of a first wavelength, and a dielectric layer in contact with the conductive layer. A size of the apertures is a size equal to or less than the first wavelength, and a ratio of a surface area of the conductive layer to a surface area of the substrate surface is within a range of equal to or greater than 36% and equal to or less than 74%. A transmissivity of the first wavelength is increased by surface plasmons induced in the apertures by light falling on the conductive layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.12/474,086 filed May 28, 2009, which claims priority to Japanese PatentApplication No. 2008-142939 filed May 30, 2008, each of which are herebyincorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an optical filter that uses localized plasmons.

2. Description of the Related Art

A hole-type optical filter in which apertures are arranged periodicallyin a thin metal film and wavelength selection is performed using surfaceplasmons is described in U.S. Pat. No. 5,973,316 and Nature, Vol. 424,14, Aug. 2003.

The transmissivity of a thin metal film having an aperture diameter of asize equal to or less than a light wavelength has been considered to beless than approximately 1%, with the specific value depending on thefilm thickness.

The description of U.S. Pat. No. 5,973,316 suggests that thetransmissivity can be increased to a certain extent by using surfaceplasmons of the thin metal film surface.

Furthermore, Nature, Vol. 424 (P824-830), 14, Aug. 2003 describes thepossibility of obtaining a RGB transmission spectrum as a hole-typeoptical filter using surface plasmons.

In a conventional optical filter using a thin metal film, lighttransmissivity is typically about several percents. However, in anoptical filter in which transmissivity is not that high, an incidentlight intensity sufficient to obtain a desired transmitted lightintensity may be high. As a result, for example, the thin metal film maybe heated as the incident light intensity rises, and structural changesmay occur in the thin metal film. This can make it impossible to obtainthe desired designed optical characteristics.

U.S. Pat. No. 5,973,316 discloses a feature for increasing thetransmissivity by using a thin metal film structural body in whichperiodic apertures are provided in a thin metal film, and matching thearrangement period of the apertures with the wavelength of surfaceplasmons propagating in the thin metal film surface.

However, in the invention disclosed in U.S. Pat. No. 5,973,316, theaperture arrangement period generally has to be matched with the plasmonwavelength, and thus there is little freedom in designing opticalcharacteristics. For example, it can be difficult to design an elementhaving desired optical characteristics in a wide-zone wavelength rangesuch as the entire visible range.

Nature, Vol. 424, 14, Aug. 2003 discloses the possibility of obtaining aRGB transmission spectrum as a hole-type optical filter, butinvestigations for increasing transmissivity and providing stablecharacteristics for such optical filters have not been conducted.

Accordingly, in the application of filters such as those describedabove, the stability of characteristics and endurance, in addition tooptical characteristics such as transmissivity and wavelength zone, arein need of improvement.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an optical filterincludes a light-shielding conductive layer provided with a plurality ofapertures on a substrate surface that selectively transmits light of afirst wavelength, and a dielectric layer in contact with the conductivelayer. A size of the apertures is a size equal to or less than the firstwavelength, and a ratio of a surface area of the conductive layer to asurface area of the substrate surface is within a range of equal to orgreater than 36% and equal to or less than 74%. A transmissivity of thefirst wavelength is increased by surface plasmons induced in theapertures by light falling on the conductive layer. According to anotheraspect of the present invention, an optical filter includes a substrate,a conductive layer with a plurality of apertures provided periodicallyon the substrate, and a dielectric layer in which the conductive layeris embedded. A size of the apertures is a size equal to or less than aresonance wavelength of the plasmon resonance so that localized surfaceplasmons are generated by a visible light falling thereon, and a maximumvalue of transmissivity at the resonance wavelength of the filter isequal to or greater than approximately 50%.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams illustrating an example of theinvention;

FIG. 2 is a graph illustrating the dependence of a light transmissionintensity on a wavelength;

FIGS. 3A and 3B are schematic diagrams illustrating an example of theinvention;

FIGS. 4A and 4B are schematic diagrams illustrating an example of theinvention;

FIG. 5 is a schematic diagram illustrating the shape of an interface inaccordance with an aspect of the invention;

FIG. 6 is a schematic diagram illustrating the shape of an interface inaccordance with another aspect of the invention;

FIG. 7 is a schematic diagram illustrating an example of the invention;

FIG. 8 is a graph illustrating the dependence of a light transmissionintensity on a wavelength;

FIG. 9 is a schematic diagram illustrating an example of the invention;

FIG. 10 is a schematic diagram illustrating an example of the invention;

FIGS. 11A and 11B are schematic diagrams illustrating an example of theinvention;

FIGS. 12A and 12B are explanatory drawings illustrating Example 1 of theinvention;

FIGS. 13A and 13B are graphs illustrating the dependence of a lighttransmission intensity on a wavelength;

FIGS. 14A to 14C are explanatory drawings illustrating Example 2 of theinvention;

FIGS. 15A to 15C are explanatory drawings illustrating Example 3 of theinvention;

FIGS. 16A to 16C are schematic diagrams illustrating a principle of theinvention;

FIG. 17 is a schematic diagram illustrating an example of the invention;

FIG. 18 is a schematic diagram of Example 4;

FIG. 19 is a schematic diagram of Example 4;

FIG. 20 is a schematic diagram of Example 4;

FIG. 21 is a schematic diagram of Example 5; and

FIG. 22 is a schematic diagram of Example 5.

DESCRIPTION OF THE EMBODIMENTS

The inventors have studied an optical filter having a structure in whichat least two metal/dielectric interfaces are disposed opposite eachother.

In a case where a smooth metal surface is in contact with a dielectric,a surface plasmon resonance (SPR) can be generated at the interfacethereof. Furthermore, even in a case where the metal surface is notsmooth, for example, in a metal nanostructure such as a metal fineparticle or metal aperture, such as in a structure of a size of equal toor less than about a light wavelength, a localized surface plasmonresonance (LSPR) can be generated.

A plasmon is a collective oscillation of free electrons inside a metalor on the surface thereof that is induced by an external electric fieldsuch as light. Since electrons bear an electric charge, the oscillationof electrons produces polarization determined by the densitydistribution of the free electrons. A phenomenon in which thispolarization couples with an electromagnetic field is called a plasmonresonance.

A resonance phenomenon of light and plasma oscillations of freeelectrons generated in a metal nanostructure is called a localizedsurface plasmon resonance (LSPR).

Thus, collective oscillations of free electrons in a metal nanostructureare induced by an external electric field such as light, theseoscillations create an electron density distribution and polarizationresulting therefrom, and an electromagnetic field that is localized inthe vicinity of the metal nanostructure is generated.

The generated LSPR absorbs and scatters light of a specific frequencywith an especially high intensity. Therefore, the transmissivity andreflectivity generated thereby depend on the wavelength.

The LSPR can be demonstrated by a metal structure with a thickness ofequal to or greater than several nanometers.

A principle according to aspects of the invention will be describedbelow using a simple structure.

Let us consider a thin metal film structural body 1611 in whichconductor/dielectric interfaces are parallel and face each other, asshown in FIGS. 16A to 16C. FIG. 16A is a view from the light incidenceside, and FIG. 16B is a side view. The conductor will be explainedhereinbelow by considering a metal. In the figure, an interface 1604 andan interface 1605 formed by a metal member 1601, a dielectric member1602, and a metal member 1603 are parallel and face each other. Thesetwo interfaces form an aperture 1606.

In the explanation above, a filter using a shape shown in FIG. 16A ismainly considered, but the filter in accordance with the invention isnot limited to this shape, and optical characteristics of the opticalfilter in accordance with aspects of the invention basically can bedemonstrated with any shape in which interfaces are disposed oppositeeach other, as shown for example in FIG. 16C.

This is believed to be because, as will be described in greater detailhereinbelow, plasmons are generated that are localized on the interface1604 and interface 1605 of the metal member (conductive layer) anddielectric member (dielectric layer).

Without being limited to any particular theory, it is believed that whenan incident light 1607 having a polarization (i.e., electric field)component shown in the figure falls on the aperture 1606, free electronsare moved and distributed, as shown by arrows in the figure, in theaperture 1606 and circumference thereof, and a charge densitydistribution appears in the vicinity of the aperture 1606. Coupling ofthe charge density distribution and electromagnetic field is a localizedplasmon.

Due to the involvement of free electrons at the periphery of aperture1606 or scattering at the interface end portions, the localized plasmonsgenerate charge density distribution at the aperture edges.

For example, in a case of an optical filter having an aperture shape,localized plasmons induced in the apertures are generated as a standingwave of a charge density wave that is reflected form an end portion 1608of the aperture 1606 shown in FIGS. 16A to 16C. As a result, theintegral multiple of half-wavelengths of plasmons induced in thisportion is approximately equal to a width (length) 1609 of theinterface.

In a case where plasmons are induced at such an interface, even when thespacing 1610 between the opposing interfaces is so small that thevisible light cannot be transmitted, the energy of this light propagatesto the outgoing side of the thin metal film structural body 1611 underthe effect of the localized plasmons.

Thus, according to one aspect of the invention, predetermined opticalcharacteristics may be demonstrated by using localized plasmons inducedin a nanosize structural body (e.g., at opposing smooth interfaces),rather than by general surface plasmons propagating along a thin metalfilm surface.

Aspects of the invention may therefore use localized plasmons that arelocalized and induced in interface portions and apertures formed by theinterfaces.

Localized plasmons are localized in the in-plane direction and also thethickness direction of a thin metal film, and where the localizedplasmons are induced at the end surface on the incidence side of theaperture, the energy of localized plasmons propagates to the end surfaceon the outgoing side of the aperture and is re-emitted in the freespace.

According to one aspect of the invention, optical characteristics of anoptical filter in which localized plasmons are induced may originate inthe shape and disposition of the nanostructure that induces thelocalized plasmons. Therefore, the transmission wavelength andtransmissivity may be controlled by accurately designing the shape anddisposition of the nanostructure.

Furthermore, the optical filter in accordance with aspects of theinvention can demonstrate the functions thereof even with one set ofopposing interfaces. This is because, as described hereinabove, inoptical characteristics of the optical filter in accordance with theinvention, the frequency of localized plasmons induced at the interfacesor the wavelength corresponding to the frequency is at least partiallydetermined by the shape of the opposing interfaces.

FIGS. 3A and 3B are schematic diagrams of an optical filter 304 in whicha thin metal film structural body 302 that has metal/dielectricinterfaces 303 of the same width and length are disposed in a dielectricsubstrate 301.

In FIGS. 3A and 3B, the optical filter is constituted by a periodicarrangement of the metal/dielectric interfaces 303. A cross-section A-A′in FIG. 3A is shown in FIG. 3B.

Likewise, an example of an optical filter of a configuration differentfrom that shown in FIGS. 3A and 3B are shown in FIGS. 4A and 4B.

FIGS. 4A and 4B are schematic diagrams of an optical filter 404 in whicha thin metal film structural body 402 that has metal/dielectricinterfaces 403 of the same width and length are disposed in a dielectricsubstrate 401.

In FIGS. 4A and 4B, the metal/dielectric interfaces form parts ofapertures that function as windows opened in a metal layer (conductivelayer) that is a light-shielding member.

In FIGS. 4A and 4B, the optical filter comprises a periodic arrangementof the apertures. A cross-section B-B′ in FIG. 4A is shown in FIG. 4B.

Because the width (length) of the metal/dielectric interface at leastpartially determines the wavelength of induced plasmons, it can play animportant role in determining the optical characteristics (such as thetransmission wavelength) of the filter.

Furthermore, while the transmissivity of a filter having the opposingmetal/dielectric interface may be affected by a variety of factors,generally the propagation loss decreases as the plasmons induced at bothinterfaces are coupled and the electromagnetic field distribution isdrawn out from the metal side to the dielectric side.

Therefore, a certain decrease in the distance between the interfaces maybe beneficial for decreasing the propagation loss. However, too small adistance between the interfaces may be undesirable because propagationloss in a case where the aperture is viewed as a plasmon waveguideincreases.

Accordingly, a surface occupancy (filling factor; referred tohereinbelow as FF) is represented by Sa/S, where Sa stands for a surfaceoccupied by a light-shielding portion in the optical filter with asurface S. Thus, the filling factor represents a ratio of surface areaof the light shielding portion (conductive layer) to a surface area ofthe substrate surface.

Furthermore, even when the apertures have an array-like arrangement, theFF can affect the transmissivity.

In the description of U.S. Pat. No. 5,973,316 mentioned above, surfaceplasmons may be actively energized by matching the aperture period of athin metal film with a wavelength of surface plasmons that propagateover a long distance along the thin metal film surface. By contrast, inthe optical filter in accordance with aspects of the invention, opticalcharacteristics may be demonstrated by inducing localized plasmonsinside the apertures. Furthermore, in certain instances inducing surfaceplasmons on the thin metal film surface, for example without inducinglocalized plasmons inside the apertures, may be undesirable.

This is because a loss inevitably occurs, as described in U.S. Pat. No.5,973,316, when surface plasmons are induced and they propagate over thethin metal film surface.

Furthermore, because the excitation conditions of surface plasmons on athin metal film surface are different from the excitation conditions oflocalized plasmons present inside an aperture, a loss may also occurwhen the surface plasmons present on the thin metal film surface arecoupled with the localized plasmons located inside the aperture. This isone more reason why transmissivity may decrease with the generation ofsurface plasmons.

However, in the optical filter in accordance with aspects of theinvention, localized plasmons may be directly induced substantially andeven entirely without converting the incident light into surfaceplasmons. Furthermore, a relatively high transmissivity can be realizedbecause coupling the localized plasmons again with the propagating lightcontributes to increase in transmissivity.

According to one aspect of the invention, a configuration in which theFF of the thin metal film in all the elements is relatively small can beconsidered as a configuration of an optical filter with a relativelyhigh transmissivity.

Furthermore, because localized plasmons are mainly induced at the edgeof apertures formed in a thin metal film, part of the energy of thelocalized plasmons propagates to the thin metal film surface surroundingthe apertures and is lost as a thermal energy inside the thin metalfilm. In a case where the FF is large, a thin metal film portion(light-shielding portion) surrounding the aperture (light-transmittingportion) is much larger than the aperture area, and the thin metal filmportion is present at a distance larger than the aperture diameter inany direction around the aperture.

As a result, the energy of localized plasmons induced in the aperturemay easily induce a charge density distribution in the thin metal filmportion surrounding the aperture, and may easily cause theabove-described energy loss.

These effects may be manifested, for example, in a case where thearrangement period of apertures matches the wavelength of surfaceplasmons, as in the filter described in the aforementioned U.S. Pat. No.5,973,316.

By contrast, in a case where the FF is small, the shape of the thinmetal film portion surrounding the aperture may be that of a fine metalwire, rather than of a flat sheet.

A mode number of surface plasmons induced on the surface of awire-shaped metal may be less than that of the thin metal film structureof the above-described flat sheet shape, because of structuralanisotropy of the wire-like metal.

As a result, the ratio at which the energy of localized plasmonpolaritons induced in the apertures is converted in the surface plasmonsof a wire-shaped thin metal film structure surrounding the apertures canbe decreased and an optical filter with a relatively small loss and hightransmissivity during transmission can be obtained.

By contrast, where the FF is too low, a spectral contrast can decreaseas the transmissivity increases. Therefore, there may be an optimum FFvalue that allows the contrast to be maintained, while also increasingthe transmissivity.

FIG. 2 illustrates an example of the dependence of transmissivityintensity on a wavelength.

FIG. 2 shows a graph of transmissivity 201 obtained in a case where Alis used as a light-shielding material and square apertures with one sideof 180 nm are provided in a square grid-like pattern with a period of350 nm (FF is about 74%), a graph of transmissivity 202 obtained in acase in which square apertures with one side of 220 nm are provided in asimilar manner (FF=60%), and a graph of transmissivity 203 obtained in acase where square apertures with one side of 280 nm are provided in asimilar manner (FF=36%). Transmissivity 204 is obtained with squareapertures with one side of 300 nm (FF is about 26%), and transmissivity205 is obtained with square apertures with one side of 156 nm (FF=80%).

As follows from FIG. 2, in a state with a higher FF and smallerapertures, the transmissivity falls almost to zero, as transmissivity205 or 201, in the foot portion of the resonance wavelength. If the FFis decreased, the maximum value of transmissivity gradually increases asin transmissivity 203 or 202.

However, the transmissivity at the foot of the resonance wavelength alsorises, eventually a flat spectral characteristic is obtained, and aspectral contrast of a configuration with a high transmissivitydecreases (204).

With consideration for light utilization efficiency in various devicesusing optical filters, a maximum value of transmissivity in a resonancewavelength of the optical filter be equal to or greater thanapproximately 50%.

Where a contrast, which is an indicator of wavelength selectivity, isconsidered, it may be that a transmissivity of a transmission spectrumin the foot portion of the spectrum is provided that is equal to or lessthan approximately 50%.

With consideration for these issues, in one version the FF may be equalto or less than about 74%, for example in order to obtain a maximumvalue of transmissivity at a resonance frequency that is approximatelyequal to or greater than 50%, and the FF may be equal to or greater thanabout 36%, for example in order to maintain the contrast of thetransmission spectrum (in other words, in order to obtain atransmissivity equal to or less than 50% in the foot portion of theresonance peak).

Thus, it may be the case that a ratio of a surface area of theconductive layer to a surface area of the substrate surface is within arange of equal to or greater than 36% and equal to or less than 74%.

Because light with a resonance wavelength is transmitted by thelocalized plasmon resonance, the transmissivity spectrum generally has acertain maximum value.

The light transmitted by the filter (i.e., element) may also includelight that is directly transmitted by the metallic light-shieldingportion, or a scattered and propagating light component that passes in avery small amount through the apertures. The wavelength dependence ofthese light components is generally determined by a cut-off of theapertures.

Therefore, the wavelength dependence has neither a maximum value nor aminimum value. Accordingly, it is clear that the optical characteristicsof the optical filter in accordance with the invention do not depend oncut-off of microapertures.

The research conducted by the inventors further demonstrated that anoptical filter in which a thin metal film structural body is embedded ina dielectric substrate is superior to a structure in which a thin metalfilm structural body is simply disposed on the dielectric substratesurface and the thin metal film structural body is exposed to the air.

Thus, where the thin metal film structural body is simply disposed onthe dielectric substrate, the frequency of plasmon resonance at aninterface in the vicinity of a boundary portion of air and the thinmetal film structural body may be different from the frequency ofplasmon resonance at an interface in the vicinity of a boundary portionof the thin metal film structural body and the dielectric substrate.

As a result, the optical spectrum width is increased or peak splittingmay occur, and characteristics that may not be desirable for an opticalfilter may develop.

In a case where the optical filter is used as a reflection filter, thereflection characteristic differs depending on whether the incidentlight falls from the dielectric substrate side or from the air side.Therefore, in order to demonstrate predetermined opticalcharacteristics, an optical filter may be provided in which light isallowed to fall only from one certain direction, and thus the degree offreedom in designing an optical system using such an optical filter canbe decreased. One more problem is that where dust or the like adheres tothe metal surface, the peak wavelength can vary.

In a case where the variations of optical characteristics caused by theaforementioned reasons are applied to a device such as environmentalsensor using a metal nanostructure, even if advantageous characteristicsare obtained, the characteristics for optical filters include stabilityand endurance and are different from those required for the sensor.

In the optical filter in accordance with aspects of the invention,energy is transmitted from the light incoming side to the light outgoingside via local plasmons localized inside the apertures, and the energyloss occurring when the energy of electromagnetic field passes insidethe apertures may be reduced.

For this purpose, it may be that an electromagnetic field distributionof local plasmons localized inside the apertures is drawn out from themetal side to the dielectric side inside the apertures by embedding adielectric (dielectric layer) with a high dielectric constant in theapertures. Such a configuration may make it possible to decrease thefraction of energy dissipated in the metal from the electromagneticfield present inside the metal.

The investigation of a structure in which a thin metal film structuralbody is embedded in the dielectric demonstrated that such a structurecan inhibit the splitting of spectral peaks, peak width enlargement, andthe decrease in transmissivity caused by the difference with the plasmonresonance frequency at the interface of air and metal.

Furthermore, such a structure can prevent the metal from oxidation,inhibit variations in optical characteristics (shift of peak wavelengthand the like) caused by adhesion of dust and the like to the metalsurface, and can improve stability and endurance.

However, where a dielectric multilayer filter or colorant filter, whichare typical optical filters, are to be used in devices, a thicknessequal to or larger than the light wavelength may be necessary, and atypical film may have a thickness of equal to or greater than about 1μm.

By contrast, the optical filter in accordance with aspects of theinvention can be configured by using a metal film (i.e., conductive thinfilm) as a light-shielding member with a thickness equal to or less thanabout 100 nm.

Even if a protective layer is laminated to a thickness of about 100 nmon the thin metal film structural body, the total thickness of theentire layer can be limited to about 200 nm. Therefore, a filter that isthinner than the conventional filters using colorants or the like can beprovided.

Therefore, where the optical filter in accordance with aspects of theinvention is used in a light-receiving element such as a CCD sensor orCMOS sensor, the light-receiving element can be miniaturized. Inaddition, an insufficiency in the quantity of received light caused bythe decrease in a point-ahead angle of pixels that results from theincrease in the number of pixels in the light-receiving element can bealleviated.

Embodiments of the invention will be described below in greater detail.

FIG. 1B is a top view of an optical filter corresponding to a firstembodiment of the invention. FIG. 1A is a cross-sectional view alongA-A′.

A light-transmitting dielectric layer 130 is provided on the surface(i.e., on a dielectric substrate surface) of a light-transmittingdielectric substrate 110, and a light-shielding metal thin-filmstructural body (i.e., electrically conductive layer) 120 is selectivelyprovided between the dielectric substrate 110 and dielectric layer 130.In other words, the dielectric layer 130 is provided on the electricallyconductive layer in contact with the electrically conductive layer.

The metal thin-film structural body (i.e., electrically conductivelayer) 120 has a first area constituting opposing interfaces 121 and 122with the dielectric layer 130 in the direction normal to the dielectricsubstrate 110, these interfaces extending along a first direction 140parallel to the surface of the dielectric substrate 110.

In another representation, the interfaces 121 and 122, which areinterfaces between the dielectric layer and metal layer, form a pair 123of interfaces at the thin metal film structural body 120, and thesepairs 123 of interfaces are provided two-dimensionally and periodicallyin an isolated state in the in-plane direction of the dielectricsubstrate 110.

In the figure, the interfaces 121 and 122 also form part of apertures125 having the function of light-transmitting windows formed in the thinmetal film structural body 120 capable of functioning as alight-shielding portion. In this case, the dielectric layer 130 isembedded also in the apertures 125. In the explanation herein, theapertures 125 have a square grid-like arrangement, but this arrangementis not limiting.

The aperture 125 has a first length 141 in a first direction 140 andalso has a second length 151 in a second direction 150 perpendicular tothe first direction 140.

In this case, the first length 141 (length of the first side in thedirection parallel to the dielectric substrate surface) and secondlength 151 (distance between the opposing first areas) are set to alength equal to or less than a wavelength of light in a visible range.

In a case where a wavelength of a localized plasmon induced at aninterface by an incident light having a polarized component (electricfield component) shown in the figure is of a lowest-order mode, thehalf-wavelength for the plasmons may be substantially equal to thelength 141 of interfaces 121 and 122. Because the size of a structure inwhich the localized plasmons can be induced by visible light is lessthan an excitation wavelength of visible light, these lengths may bemade equal to or less than a wavelength of light in a visible range.

Here, as an example, the apertures 125 have a square shape in which thefirst length 141 is equal to the second length 151, with one side being240 nm. The square shape may be provided from the standpoint offacilitating the design of optical characteristics, but other polygonalshapes may be also used. Furthermore, the aperture may also have a roundor elliptical shape. For example, a regular polygonal shape or roundshape may be advantageous because polarization dependence can beinhibited. A round shape may be advantageous because production may berelatively easy and production accuracy can be easily maintained.

For example, in the case of square apertures, the reference numeral 501shown in FIG. 5 denotes the first length, and the reference numeral 502denotes the second length.

Where the aperture provided with an interface pair (opposing interfaces)123 has a regular polygonal shape, the first length (length of the firstarea in the direction parallel to the substrate surface) is taken as alength indicated by the reference numeral 601, and the second length(distance between the first areas) is taken as a length indicated by thereference numeral 602, as shown for example in FIG. 6,

Furthermore, in accordance with an aspect of the invention, where theapertures have a round shape, the length of the first area in thedirection parallel to the substrate surface and the distance between theopposing first areas can indicate the circle diameter.

In a case where an aperture size is referred to in the presentdescription, it can be taken to indicate the above-described first orsecond length of the aperture, the diameter of a round aperture, and thediagonal length of a polygonal aperture.

For example, with a maximum wavelength of transmitted light being in ared zone (this is equal to or greater than 600 nm and equal to or lessthan 700 nm as the first wavelength), the aperture size can be taken asa size equal to or less than the first wavelength.

In the present embodiment, because of a resonance of the interface pair123 and light falling on the dielectric substrate or dielectric layer,the transmissivity of a predetermined wavelength in the visible lightregion can be increased by localized surface plasmons induced at thesurface of interfaces 121 and 122. In other words, the transmitted lightof the predetermined wavelength may be preferentially (e.g.,selectively) generated.

In the group of apertures (set of apertures 125) shown for example inFIGS. 1A and 1B, a period 145 and a period 155 with which the interfacepairs 123 are provided may be made equal to or less than a wavelength oflight in the visible region. This is because in a case where theaperture arrangement period is larger than the wavelength region of thelight of interest, high-order diffracted light can be generated and theintensity of zero-order diffracted light can decrease.

Furthermore, in one version the period 145 and period 155 with which theinterface pairs 123 are provided may be less than the resonancewavelength of plasmons induced in the apertures. Where the apertureperiod becomes close to the plasmon resonance wavelength, the so-calledWood anomaly occurs, the shape of a peak produced by the plasmonresonance can change significantly from that of the Lorentz type, andcharacteristics that are different from the intended and preselectedoptical characteristics can be demonstrated.

The Wood anomaly, as referred to herein, is a phenomenon in which theincident light is diffracted by a periodic structure and the diffractedlight propagates in the extreme vicinity of the surface of the metalperiodic structure and parallel to the surface, thereby increasing lossand decreasing the refraction efficiency.

Assuming, as an example, that a plasmon resonance is generated in a redwavelength zone, the periods 145 and 155 of 350 nm may be taken.

In one version, the thickness (i.e., layer thickness) 160 of the thinmetal film structural body (i.e., conductive layer) 120 may be equal toor less than the light wavelength in the visible light region.

Because the length 141 and interface spacing 151 may be equal to or lessthan the light wavelength in the visible light region, where too large athickness of the metal thin film structural body is set in themicroprocessing process used in the production of the optical filter inaccordance with the invention, the structure may be difficult to produceand the production error may become large. Accordingly, the thickness ofthe thin metal film (conductive layer) is herein assumed to be 60 nm byway of example.

Aluminum, gold, silver, platinum, and the like can be used as a materialconstituting the thin metal film structural body 120. Among them,aluminum has a plasma frequency higher than that of silver and may makeit possible to relatively easily design a filter with opticalcharacteristics that physically include the entire visible range (Ag:about 3.8 eV (about 325 nm); Al: about 15 eV (about 83 nm)). In oneversion, the metal film structural body 120 (i.e., conductive layer) maycomprise Al or an alloy or a compound including Al.

Furthermore, because aluminum is chemically more stable than silver andthe like, the predetermined optical characteristics can be demonstratedwith good stability for a long time.

Moreover, because aluminum has an imaginary part of dielectric constanthigher than that of silver, sufficient light shielding ability can bedemonstrated even when the film thickness of aluminum is less than thatof silver. In addition, microprocessing of aluminum may be easier thanthat of silver.

Further, because aluminum, like platinum, is chemically extremelyinactive, the use thereof generally causes no inconveniences such asdifficult microprocessing in dry etching.

The thin metal film structural body (dielectric layer) 120 may be frommixtures, alloys, and compounds including aluminum, gold, silver, andplatinum.

A material for the dielectric substrate 110 can be appropriatelyselected from materials with a high transmissivity of visible light,such as at least one of quartz (silicon dioxide), metal oxides such astitanium dioxide, and silicon nitride, which are materials that transmitlight in a visible region. Furthermore, polymer materials such aspolycarbonates and polyethylene terephthalate also can be used for thedielectric substrate 110.

A dielectric is herein indicated as the substrate material, but thesubstrate in accordance with the invention is not limited todielectrics.

The substrate in accordance with the invention is a member that supportsthe light-shielding conductive layer provided with apertures.

For example, in a case where an optical filter is incorporated byforming a sensor portion (a CMOS sensor or the like) having aphotoelectric conversion portion on a silicon wafer, then laminating awiring layer, an insulating layer, and the like, and then laminating aconductive layer, the silicon wafer can be called the substrate. Anintermediate insulating layer or the like also can be taken as thesubstrate. Therefore, the substrate in accordance with the inventionincludes support bodies of a comparatively large thickness, such as adielectric substrate and a semiconductor substrate, and also supportbodies of a comparatively small thickness, such as a semiconductivelayer and an insulating layer.

Similarly to the dielectric substrate 110, a material of the dielectriclayer 130 can comprise any one or more of quartz (silicon dioxide),titanium dioxide, silicon nitride, and the like. In a case ofincorporating in a semiconductor device such as a CMOS sensor, a typicalinsulating film that is used in a semiconductor fabrication process canbe used. Furthermore, a polymer material such as a polycarbonate orpolyethylene terephthalate can be also used as the material for thedielectric layer 130.

In one version, the substrate 110 may comprise a dielectric, and thedifference in dielectric constant between the substrate and thedielectric layer 130 may be equal to or less than 5%.

This is because where the dielectric constant of the dielectricsubstrate 110 is significantly different from the dielectric constant ofthe dielectric layer 130, there can be a large difference between anexcitation wavelength of plasmons at the end portions of interfaces 121and 122 at the side of the dielectric substrate 110 and the excitationwavelength of plasmons at the end portions of interfaces 121 and 122 atthe side of the dielectric layer 130.

Thus in this case, a peak at an unexpected resonance wavelength, orincrease in peak width, can occur.

Accordingly, in one version the dielectric constant of the dielectricsubstrate may be identical to the dielectric constant of the dielectriclayer.

The optical filter in accordance with aspects of the invention may havea laminated configuration in which a plurality of layers of the thinmetal film structural body are laminated in the dielectric layer.

Optical characteristics of the laminated element, and more particularlya transmission spectrum thereof, will be described below.

In a case where the interlayer distance during lamination is equal to orgreater than a distance (typically about 100 nm) that is reached by thenear-field distribution of localized plasmons induced in each thin metalfilm structural body, a spectrum of the product of transmission spectraof the layers of the thin metal film structural body may occur. This maybe because optical characteristics of each thin metal film structuralbody are maintained since no near-field interaction occurs between thethin metal film structural bodies. In this embodiment, opticalcharacteristics of the entire element may be comparatively easy todesign.

By contrast, where the aforementioned distance between the thin metalfilm structural bodies is equal to or less than 100 nm, the localizedplasmons induced in the thin metal film structural bodies will interactwith each other. As a result, optical characteristics can becomecomplex.

By contrast with the case in which the aforementioned interaction isabsent, various changes such as splitting or broadening of transmissionspectrum peaks, and decrease or increase in transmissivity, may occur.Although optical characteristics of the entire element may be difficultto design in this case, it may be possible to form a spectrum shape thatis more complex than that of a single-layer configuration.

Referring to FIG. 7, a first thin metal film structural body 702 isformed on a dielectric substrate 701, and then a first dielectric layer703 is coated. A second thin metal film structural body 704 is disposedon the first dielectric layer 703, and a second dielectric layer(another dielectric layer) 705 is formed on the second thin metal filmstructural body.

As a result, for example, by laminating a two-layer optical filter R, itmay be possible to obtain a transmission spectrum that has a finer linewidth than that of a single-layer configuration.

The configurations of the first thin metal film structural body 702 andthe second thin metal film structural body 704 are not limited to thosewith an identical arrangement period of apertures 706, or those with anidentical shape of apertures.

In the laminated optical filter of the present embodiment, from thestandpoint of ease of design, it may be that lamination is performedwith a lamination spacing that causes practically no near-fieldinteraction. For example, a lamination spacing equal to or greater than100 nm may be provided.

(Computation Results)

FIG. 8 shows a graph representing the results obtained by using theabove-described structure and conducting numerical computations. Thisgraph shows the dependence of transmission intensity on wavelength.

In the filter used herein, aluminum was used as the thin metal filmstructural body, the aperture diameter was set to 240 nm, the period was350 nm, and the thickness was 60 nm. The transmission spectrum of theoptical filter is like a transmission spectrum 801, and the opticalfilter functions to transmit light with a wavelength close to 650 nmwith high intensity.

Because the wavelength of 650 nm is within a red zone, a word “Red” isused to denote the optical filter R. Because this optical filter Rtransmits a red color wavelength, the filter can be used as a redprimary color filter.

The wavelength of the transmission spectrum, the spectral width, and thetransmissivity can be varied by varying the aperture diameter or period.

For example, an optical filter having a transmission spectrum 802 with amaximum of transmissivity close to green of a visible range (wavelength550 nm) can be constituted by taking an aperture diameter of 200 nm, aperiod of 280 nm, and a thickness of 60 nm. This filter will be referredto as an optical filter G. The optical filter G can be used as a greenprimary color filter.

Likewise, an optical filter having a transmission spectrum 803 with amaximum of transmissivity close to blue in a visible range (wavelength450 nm) can be constituted by taking an aperture diameter of 160 nm, aperiod of 230 nm, and a thickness of 60 nm. This filter will be referredto as an optical filter B. The optical filter B can be used as a blueprimary color filter.

In the reflection spectrum of the optical filter of the presentembodiment, reflectivity is minimal in the vicinity of a wavelength atwhich transmissivity is maximal. The optical filter of the presentembodiment can thus be used not only as a transmission filter, but alsoas a reflection filter.

(Design Guidelines)

The relationship between parameters constituting the thin metal filmstructural body and optical characteristics will be explained below.

The localized plasmon resonance induced in the apertures is a chargedensity distribution accompanying plasma oscillations of free electronsat the interfaces, and this charge density distribution or opticalcharacteristics of the apertures can be affected by the shape ofapertures.

For example, where a length of apertures in the direction perpendicularto a polarization direction is increased, while maintaining a constantlength of the apertures in the polarization direction of lightilluminating the apertures, the thickness of metal layer, and thearrangement period of apertures, the resonance wavelength will shift tolonger wavelengths. Furthermore, not only will the resonance wavelengthshift to the longer wavelengths, but the peak width and transmissivityin the transmission peak will also increase.

Thus, it is clear that in order to generate a longer wavelength oflocalized plasmon resonance in the apertures, the length of apertures inthe direction perpendicular to the polarization direction may beincreased. Polarization of light falling on the optical filter may notbe required to be strictly parallel to the normal direction of theapertures.

In a state in which the length of apertures in the directionperpendicular to the polarization direction of light illuminating theapertures, the thickness of metal layer, and the arrangement period ofapertures are constant, the resonance wavelength shifts to shorterwavelengths with the increase in the length of apertures in thepolarization direction. Furthermore, the peak width increases andtransmissivity at the resonance wavelength also increases.

In a state in which the length of apertures in the polarizationdirection of light illuminating the apertures, the length of aperturesin the direction perpendicular to the polarization direction, and thearrangement period of apertures are constant, the increase in metallayer thickness may practically not change the resonance wavelength, butthe transmissivity at the resonance wavelength and resonance widthdecrease.

In a state in which the length of apertures in the polarizationdirection of light illuminating the apertures, the length of aperturesin the direction perpendicular to the polarization direction, and themetal layer thickness are constant, the increase in the aperturearrangement period tends to shift the resonance wavelength to longerwavelengths, decrease the transmissivity at the resonance wavelength,and decrease the resonance width.

Based on these findings, the parameters such as aperture shape andaperture arrangement period can be optimized, and an optical filterhaving the predetermined resonance wavelength can be designed.

The results of investigation conducted by the inventors demonstratedthat in order to obtain a resonance wavelength of an optical filter in ared zone, that is, within a wavelength range of equal to and higher than600 nm and equal to or less than 700 nm (i.e., a maximum value of atransmission spectrum within the wavelength range), the aperturediameter may be set in a range of equal to or greater than 220 nm andequal to or less than 270 nm. Furthermore, the thickness of the thinmetal film structural body may be set in a range of equal to or greaterthan 10 nm and equal to or less than 200 nm, and the aperturearrangement period may also be set within a range of equal to or greaterthan 310 nm and equal to or less than 450 nm.

In order to obtain a resonance wavelength of an optical filter in agreen zone, that is, within a wavelength range of equal to or greaterthan 500 nm and lower than 600 nm (i.e., a maximum value of atransmission spectrum within the wavelength range), the aperturediameter may be set in a range of equal to or greater than 180 nm andlower than 220 nm.

Furthermore, the thickness of the thin metal film structural body may beset in a range of equal to or greater than 10 nm and equal to or lessthan 200 nm, and the aperture arrangement period may also be set withina range of equal to or greater than 250 nm and equal to or less than 310nm.

In order to obtain a resonance wavelength of an optical filter in a bluezone, that is, within a wavelength range of equal to or greater than 400nm and lower than 500 nm (i.e., a maximum value of a transmissionspectrum within the wavelength range), the aperture diameter may be setin a range of equal to or greater than 100 nm and lower than 180 nm.

Furthermore, the thickness of the thin metal film structural body may beset in a range of equal to or greater than 10 nm and equal to or lessthan 200 nm, and the aperture arrangement period may be set within arange of equal to or greater than 170 nm and equal to or less than 250nm.

In a second embodiment an RGB filter with Bayer arrangement will beexplained.

As shown in FIG. 9, for example, the above-described optical filter R(e.g., transmission spectrum 801) is disposed in a region 901, theoptical filter G (e.g., transmission spectrum 802) is disposed in aregion 902, and the optical filter B (e.g., transmission spectrum 803)is disposed in a region 903. By using the filter in accordance withaspects of the invention with such a disposition, it may be possible toconfigure a color filter with a Bayer arrangement. In the presentembodiment, the aperture shape is different in each region and theaperture arrangement period is also different, but such a configurationis not limiting. For example, aperture groups that differ only in theaperture period may be disposed in the regions. Alternatively, aperturegroups that differ only in the size of apertures may be disposed in theregions.

In other words, there may be two or more first aperture groups, thefirst apertures may be provided with mutually different periods, and thefirst aperture groups may be disposed in mutually different regions ofthe dielectric substrate surface.

A second aperture group including second apertures of a shape differentfrom that of the first apertures constituting the first aperture groupmay be disposed in the regions. Thus, the second aperture has a firstlength in a first direction and a second length in a second direction,and the first length of the second aperture differs from the firstlength of the first aperture, or the second length of the secondaperture differs from the second length of the first aperture. As aresult, the second aperture group may be capable of increasing thetransmissivity of light at a wavelength (second wavelength) differentfrom the resonance wavelength (first wavelength) of the first aperturegroup.

FIG. 10 illustrates a third embodiment in which square apertures arearranged in a triangular grid. In a case where a triangular gridarrangement is used, unit vector components of the grid may not beorthogonal. Therefore, the dependence of optical characteristics of thefilter on incident light polarization, or spectral variations caused byoblique irradiation, can be inhibited to a greater extent than in thesquare grid arrangement.

Such a triangular grid arrangement can be also represented as anarrangement in which a plurality of apertures arranged in an orthogonalgrid are disposed in overlapping regions.

Thus, a first aperture group 1002 (dot lines in the figure) and a secondaperture group 1003 (dot-dash line in the figure) constituted by firstapertures 1001 can be represented as being disposed in overlappingregions.

In a fourth embodiment, an example will be explained in which, similarlyto the third embodiment, a plurality of aperture groups are disposedwith overlapping.

FIG. 11A shows an example in which first aperture groups that havedifferent periods are disposed in overlapping regions. First apertures1101 constituting the first aperture group 1102 (solid lines in thefigure) are provided with a period 1103, and first apertures 1104constituting a second aperture group 1105 are provided with a period1106. In the present embodiment, because the arrangement periods ofapertures are mutually different, an optical filter can be configuredthat has optical characteristics inherent to the two aperture groups.

Thus, the optical filter shown in FIG. 11A has two or more firstaperture groups in the in-plane direction of the dielectric substrate,and the arrangement periods of the first apertures constituting the twoor more first aperture groups are mutually different. Furthermore, thesetwo or more first aperture groups are disposed in overlapping regions.

FIG. 11B shows an example in which a first aperture group and a secondaperture group are disposed in overlapping regions. First apertures 1107constitute a first aperture group 1108, and second apertures 1109constitute a second aperture group 1110. Because the aperturesconstituting the aperture groups are different, optical characteristicsinherent to the two aperture groups can be demonstrated at the sametime.

Thus, the optical filter shown in FIG. 11B has, separately from thefirst aperture group, the second aperture group in which a plurality ofsecond apertures are provided two-dimensionally and periodically in anisolated state in the in-plane direction of the dielectric substrate.The second aperture has a first length in a first direction and a secondlength in a second direction, and the first length and second length areequal to or less than a light wavelength in a visible region. The firstlength of the second aperture is different from the first length of thefirst aperture, or the second length of the second aperture is differentfrom the second length of the first aperture, and the first aperturegroup and second aperture group are disposed in overlapping regions. Asa result, the resonance wavelength (first wavelength) of the firstapertures is different from the resonance wavelength (second wavelength)of the second apertures.

Such an embodiment also includes, for example, an optical filter (e.g.,FIG. 17) in which apertures 1703 are arranged with a plurality ofperiods (period A 1701 and period B 1702) in a plane. FIG. 17 shows anexample of arrangement in which a square grid has a plurality ofperiods, but the number of periods, etc., of this embodiment are notlimiting.

The invention will be described hereinbelow in greater detail on thebasis of specific examples thereof, but it should be understood that theinvention is not limited to these examples.

Example 1

Monolayer Structure

A method for producing a RGB transmission filter and opticalcharacteristics of the filter will be described below.

FIG. 12A is a schematic diagram illustrating a state in which aluminumis vapor deposited as a thin metal film layer 1202 to a thickness of 60nm on the surface of a dielectric substrate 1201 composed of a quartzsubstrate with a thickness of 525 μm and a resist 1203 for electron beam(EB) lithography is coated on the thin metal film layer. A method forforming the thin metal film layer 1202 is not limited to only vapordeposition, and may also be sputtering or the like.

The resist 1203 is then patterned by using an EB lithography device. Theresist pattern is produced to a shape such that square apertures withone side of about 240 nm are arranged in a square grid with a period ofabout 350 nm. A thin metal film structural body 1204 can be formed bydry etching with plasma of a gaseous mixture of chlorine and oxygen byusing the resist pattern as an etching mask. The dry etching gas is notlimited to chlorine and oxygen, and argon or other gases may be alsoused.

A method for fabricating the etching mask is not limited to EBlithography, and photolithography or the like may be also used.Furthermore, patterning of the thin metal film layer 1202 may be alsoperformed by forming a resist pattern by EB lithography orphotolithography on the dielectric substrate 1201, forming the thinmetal film layer 1202, and then using a lift-off process. When thelift-off process is used, the negative-positive inversion has to beperformed with respect to the above-described process.

The thin metal film layer 1202 may be also directly processed by using afocused ion beam processing device (FIB processing device).

A thin quartz film is then formed by sputtering to a thickness of 300 nmas a dielectric layer 1205 on the thin metal film structural body 1204.The optical filter thus formed is shown in FIG. 12B. The film formationmethod is not limited to sputtering, and the film can also be formed byCVD, or can be coated by a SOG (Spin On Glass) method. HSG (hydrogenatedsilsesquioxane) is an example of inorganic SOG and MSQ (methylsilsesquioxane) is an example of organic SOG.

FIG. 13A shows a transmission spectrum of the optical filter produced inthe above-described manner. A transmission spectrum R denoted by thereference numeral 1301 was found by numerical computations. It is clearthat this filter has a maximum value of transmissivity in the vicinityof a wavelength of 650 nm. Because the wavelength at which thetransmission peak is shown corresponds to a red color of a visiblerange, the filter functions as a primary color filter that transmits redlight.

A transmission spectrum G denoted by the reference numeral 1302 isobtained by providing a thin metal film structural body 1204 with anaperture diameter of about 200 nm, a thickness of about 60 nm, and anaperture arrangement period of about 280 nm. Likewise, a transmissionspectrum B denoted by the reference numeral 1303 is obtained with anaperture diameter of about 160 nm, a thickness of about 60 nm, and anaperture arrangement period of about 230 nm. These optical filterstransmit RGB, respectively, and function as primary color filters.

The reflection spectrum of the filter of this example has a minimumreflectivity at a wavelength almost equal to a wavelength at which thetransmissivity has a maximum.

Therefore, by using the optical filter of the present example as areflection filter, as shown in FIG. 13B, it is possible to obtain areflection spectrum R denoted by the reference numeral 1304 from thefilter having a transmission spectrum R. Likewise, the filter having atransmission spectrum G can produce a reflection spectrum G denoted bythe reference numeral 1305, and the filter having a transmissionspectrum B makes it possible to obtain a reflection spectrum B denotedby the reference numeral 1306. Thus, these optical filters can functionas optical filters that intensively reflect auxiliary colors (cyan,magenta, yellow) of red, green, and blue of the visible range.

The present example is explained by using a configuration in whichsquare apertures are arranged in a square grid in a thin metal filmstructural body, but a triangular grid arrangement may be also used.With the triangular grid arrangement, the dependence of incident lightpolarization can be inhibited and oblique incidence characteristic maybe improved. Furthermore, the aperture shape is not limited to square,and may also be, for example, a regular rectangle or a circle.

Example 2

Bayer Configuration

A method for producing a RGB transmission filter of a Bayerconfiguration and optical characteristics of the filter will bedescribed below.

FIG. 14A is a schematic diagram illustrating a state in which aluminumis vapor deposited as a thin metal film layer 1402 to a thickness of 60nm on the surface of a dielectric substrate 1401 composed of a quartzsubstrate with a thickness of 525 μm and a resist 1403 is coated on thethin metal film layer.

The resist 1403 is then patterned by using an EB lithography device. Theresist pattern shape is such that a patterned square shape in whichsquare apertures with one side of about 240 nm are arranged in a squaregrid with a period of about 350 nm has a side of about 10 μm, and such apatterned square shape is taken as a pattern portion A 1404.

A square shape in which square apertures with one side of about 200 nmare arranged in a square grid with a period of about 280 nm is taken asa pattern portion B 1405, and a square shape in which square apertureswith one side of about 160 nm are arranged in a square grid with aperiod of about 230 nm is taken as a pattern portion C 1406.

A structure in which these pattern portions are disposed with a spacingof 10 μm therebetween as shown in FIG. 14B is produced. A thin metalfilm structural body 1407 is produced by dry etching with plasma of agaseous mixture of chlorine and oxygen.

The shape of apertures is not limited to a square shape and may bepolygonal or round.

A thin quartz film with a thickness of 500 nm is formed as a dielectriclayer 1408 by sputtering on the thin metal film structural body 1407.The optical filter thus formed is shown in FIG. 14C. FIG. 14C is an A-A′section in FIG. 14B.

A light-shielding layer may be formed to prevent color mixing betweenthe above-described pattern portions. Furthermore, where the thicknessof thin metal film structural bodies constituting pattern portions isthe same, as in the present example, these pattern portions can befabricated in the same process and boundaries between the patternportions can be eliminated.

The pattern portions A, B, and C produced in the above-described mannerhave a transmission spectrum R indicated by the reference numeral 1301,a transmission spectrum G indicated by the reference numeral 1302, and atransmission spectrum B indicated by the reference numeral 1303, asshown for example in FIG. 13A. These pattern portions can function asrespective RGB primary color filters.

Furthermore, where all the pattern portions are produced to have thesame thickness, as in the present example, RGB primary color filters canbe produced in the same batch.

Therefore, a process for separate coating of three colors RGB to producea Bayer arrangement structure with the conventional color filters usingcolorants may become unnecessary, the production process time can beshortened, and the production process can be simplified. Separatecoating of three colors RGB is not limited to Bayer arrangement and maybe performed when any color filter is configured using differentcolorants.

Example 3

Hole Lamination

A method for producing a laminated filter and optical characteristics ofthe filter will be described below.

FIG. 15A is a schematic diagram illustrating a state in which aluminumis vapor deposited as a thin metal film layer 1502 to a thickness of 60nm on the surface of a dielectric substrate 1501 composed of a quartzsubstrate with a thickness of 1 mm and a resist 1503 for electron beam(EB) lithography is coated on the thin metal film layer.

The resist 1503 is then patterned by using an EB lithography device. Theresist pattern shape is obtained by arranging square apertures with oneside of about 240 nm in a square grid with a period of about 350 nm. Afirst thin metal film structural body 1504 is produced by dry etchingwith plasma of a gaseous mixture of chlorine and oxygen.

A thin quartz film with a thickness of 300 nm is formed as a firstdielectric layer 1505 on the first thin metal film structural body 1504.The thickness of the first dielectric layer 1505 is not limited to 300nm, but an interlayer distance may be provided that substantially doesnot affect the near-field interaction with a second thin metal filmstructural body that will be produced in the next process.

Then, as shown in FIG. 15B, aluminum is vapor deposited as a second thinmetal film layer 1506 to a thickness of 60 nm on the surface of thefirst dielectric layer 1505. Thus, a plurality of conductive layers aredisposed on the dielectric substrate. Then a resist for electron beam(EB) lithography is coated as a resist layer on the second thin metalfilm layer 1506. The resist layer is then patterned by using an EBlithography device. The resist pattern shape is obtained by arrangingsquare apertures with one side of about 240 nm in a square grid with aperiod of about 350 nm. A second thin metal film structural body 1507 isproduced by dry etching with plasma of a gaseous mixture of chlorine andoxygen by using the resist pattern as an etching mask.

Then, as shown in FIG. 15C, a thin quartz film with a thickness 400 nmis formed by sputtering as a second dielectric layer 1508 on the secondthin metal film structural body 1507.

The transmission spectrum of the first thin metal film structural bodyand second thin metal film structural body of the filter has a peakwavelength close to about 650 nm, and the transmission spectrum of thesecond thin metal film structural body also has a peak wavelength closeto about 650 nm. As a result, optical characteristics of the laminatedfilter of the present example are close to optical characteristicsrepresenting a product of the transmission spectrum of the first thinmetal film structural body and the transmission spectrum of the secondthin metal film structural body. Therefore, the laminated filter of thepresent example functions as an optical filter that transmits red in aband that is narrower than that of a monolayer filter.

Example 4

The present example relates to a light detection element using any ofthe optical filters explained in Examples 1 to 3, an image pickupelement in which light detection elements are arranged in an array, anda digital camera incorporating the image pickup element.

FIG. 18 is a schematic diagram of a light detection element using theoptical filter of the invention.

In a light detection element 1807, light falling from the outsidethrough a microlens 1801 is introduced in a photoelectric conversionunit 1805. In the photoelectric conversion unit, an electric chargecorresponding to the incident light is generated. In addition to thephotoelectric conversion unit 1805, the light detection element includesan optical filter 1802 disclosed in the invention, a dielectric layer1803, an electric circuit unit 1804, and a semiconductor substrate 1806.The optical filter 1802 includes a structure in which a plasmonresonance can be induced in response to the falling light, as in themetal structural body 120 shown in FIGS. 1A and 1B.

A method for producing such a light detection element will be describedbelow.

First, the photoelectric conversion unit 1805 is formed on thesemiconductor substrate 1806, and the electric circuit unit 1804 ispatterned from above by using photolithography or the like. A process offorming the dielectric layer 1803 is then repeated, thereby forming thepredetermined number of electric circuit layers and dielectric layers. Ametal layer is then formed from above and apertures are patterned byusing a microprocessing device such as an electron beam lithographydevice, thereby forming the optical filter 1802. A light detectionelement using the optical filter of the invention can be thereafterproduced by forming the microlens 1801 from above by using a resin orthe like.

In this configuration, the optical filter is disposed directly below themicrolens, but the disposition area of the optical filter is not limitedby such a configuration. For example, the optical filter may belaminated directly above or close to the photoelectric conversion layer,or between the electric circuit layers. In order to generate a plasmonresonance effectively with the optical filter even in a case where theoptical filter is located directly above the photoelectric conversionlayer, a structure may be provided in which a thin electricallyinsulating layer is provided between the optical filter andphotoelectric conversion layer to provide electric insulation. As aresult, the energy of plasmon resonance can be substantially preventedfrom dissipating to the semiconductor substrate or photoelectricconversion layer. By thus bringing the optical filter close to thephotoelectric conversion portion, it may also be possible to detect withgood efficiency a scattered light component produced by the opticalfilter with the photoelectric conversion portion

FIG. 19 is a schematic diagram of an image pickup element using theoptical filter of the invention.

In FIG. 19, a plurality of the above-described light detection elements(pixels) 1901 are arranged as a 3 rows×3 columns two-dimensional matrixin a pixel area 1900. In FIG. 19, the pixel area 1900 is in the form ofa 3 rows×3 columns two-dimensional matrix, but it can be also, forexample, a 7680 rows×4320 columns matrix. In FIG. 19, numerals 1902 and1903 denote pixels as well.

Referring to FIG. 19, a vertical scan circuit 1904 and a horizontal scancircuit 1905 serve to select and read the optical detection elements(pixels) disposed in the pixel area 1900.

FIG. 20 is a schematic diagram of a digital camera incorporating theimage pickup element configured as shown in FIG. 19.

In FIG. 20, the reference numeral 2001 stands for a camera body, 2007—aneye lens, 2008—a shutter, and 2009—a mirror.

The image pickup element according to the invention is denoted by thereference numeral 2004, and the light falls on the image pickup element2004 via a pickup optical system (lens) 2002 disposed inside a lensbarrel 2003.

As a result, an electric charge is generated in each pixel of the imagepickup element 2004 correspondingly to a pickup object, and the pickupobject can be reproduced correspondingly to the generated charges. Thepickup object can be reproduced at a monitor display device 2005 orrecorded on a recording medium 2006 such as a memory card.

Because the thickness of the optical filter in accordance with theinvention is less than that of a typical color filter configured bycolorants, the image pickup element in accordance with the inventionthat is shown herein can be configured to have a small thickness. As aresult, the distance from the surface of the image pickup element to thephotoelectric conversion portion of the image pickup element isdecreased and, therefore, light utilization efficiency is increased. Asa consequence, sensitivity of the image pickup element in accordancewith the invention can be increased.

Example 5

A spectral element using any of the optical filters explained inExamples 1 to 3 will be described below with reference to FIG. 21.

An optical filter layer 2102 is disposed on a line sensor 2103 having aphotoelectric conversion layer 2101 arranged thereon in aone-dimensional fashion. The optical filter layer 2102 has apertures2104 that differ in size or shape correspondingly to pixels located inthe line sensor. Where the size or shape of the apertures 2104 isdifferent, the transmissivity spectrum shapes of apertures will also bedifferent. As a result, for example, where the apertures differ in size,the wavelength at which the light reception efficiency in each pixel ofthe element of the present example is the highest will differ among thepixels. Therefore, by providing the optical filter layer having theabove-described structure on a line sensor, the element of the presentexample can configure a spectral detector that combines a spectralfunction and a light detection function.

With the structure in which the optical filter layer is disposeddirectly on the light sensor, as in the present example, it is possibleto miniaturize the spectral element.

In this example, a sensor of a one-dimensional structure is configuredby arranging the optical filter on a line sensor, but an optical filterlayer may be also disposed on a two-dimensional area sensor, as shownfor example in FIG. 22.

An optical filter layer 2202 is disposed on an area sensor 2203 in whicha photoelectric conversion layer 2201 is arranged in a two-dimensionalfashion. The optical filter layer 2202 has apertures 2204 that differ insize or shape correspondingly to pixels located in the areas sensor.Where the size or shape of apertures 2204 are different, thetransmissivity spectra of apertures and dependence thereof onpolarization will be also different. As a result, by using rectangularapertures such as shown in FIG. 22, it may be possible to impart thetransmission spectrum with dependence on polarization. In this case,rectangular apertures with different angular orientations can bedisposed in the plane, as shown in FIG. 22, so that the apertures willdiffer in an angle formed by a longitudinal direction of rectangularaperture and incident light polarization. Furthermore, where theapertures differ in size, in addition to the incident lightpolarization, the polarization and wavelength at which the lightreception efficiency is the highest will differ among the pixels of theelement of the present example. Therefore, in a case where the opticalfilter layer having the above-described structure is provided on thearea sensor, the element of the present example can configure a spectralpolarization detector that combines a spectral function and apolarization detection function. With the element of the presentexample, it is possible to produce a small-size element that can acquirelight spectrum information and polarization information at the sametime.

The invention can thus be applied not only to an optical filter, butalso to various devices using the same.

Accordingly, the examples according to the present invention demonstratethat a high-endurance optical filter can be provided that has arelatively high transmissivity in a visible light region, and that candemonstrate substantially stable characteristics over a relatively longperiod of time. Furthermore, the examples demonstrate that it ispossible to provide an optical filter with a small film thickness inwhich the transmission spectrum has a primary color characteristic.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2008-142939, filed May 30, 2008, which is hereby incorporated byreference herein in its entirety.

1. An optical filter comprising: a substrate; and a metal layer providedwith a plurality of apertures on the substrate surface that selectivelytransmits light of a first wavelength, wherein a size of the aperturesis a size equal to or less than the first wavelength, and a ratio of asurface area of the metal layer to a surface area of the substratesurface is within a range of equal to or greater than 36% and equal toor less than 74%, and wherein a transmissivity of the first wavelengthis increased by localized plasmon resonance induced by light falling onthe metal layer.
 2. The optical filter according to claim 1, wherein thefirst wavelength is in a visible range.
 3. The optical filter accordingto claim 1, wherein the apertures are of round or regular polygonalshape.
 4. The optical filter according to claim 1, wherein a thicknessof the conductive layer is equal to or less than a wavelength of lightin a visible range.
 5. The optical filter according to claim 1, whereinthe conductive layer comprises Al or an alloy or a compound includingAl.
 6. The optical filter according to claim 1, wherein the substratecomprises a dielectric.
 7. The optical filter according to claim 6,wherein the dielectric comprises any one or more of silicon dioxide,titanium dioxide, and silicon nitride.
 8. The optical filter accordingto claim 1, wherein a diameter of the aperture is within a range ofequal to or greater than 220 nm and equal to or less than 270 nm, athickness is within a range of equal to or greater than 10 nm and equalto or less than 200 nm, an arrangement period is within a range of equalto or greater than 310 nm and equal to or less than 450 nm, and amaximum value of a transmission spectrum is demonstrated within awavelength range of equal to or greater than 600 nm and equal to or lessthan 700 nm.
 9. The optical filter according to claim 1, wherein adiameter of the aperture is within a range of equal to or greater than180 nm and equal to or less than 220 nm, a thickness is within a rangeof equal to or greater than 10 nm and equal to or less than 200 nm, anarrangement period is within a range of equal to or greater than 250 nmand equal to or less than 310 nm, and a maximum value of a transmissionspectrum is demonstrated within a wavelength range of equal to orgreater than 500 nm and equal to or less than 600 nm.
 10. The opticalfilter according to claim 1, wherein a diameter of the aperture iswithin a range of equal to or greater than 100 nm and equal to or lessthan 180 nm, a thickness is within a range of equal to or greater than10 nm and equal to or less than 200 nm, an arrangement period is withina range of equal to or greater than 170 nm and equal to or less than 250nm, and a maximum value of a transmission spectrum is demonstratedwithin a wavelength range of equal to or greater than 400 nm and equalto or less than 500 nm.
 11. The optical filter according to claim 1,wherein the apertures are arranged with a plurality of periods in thein-plane direction of the substrate.
 12. The optical filter according toclaim 1, wherein a plurality of metal layers are disposed on thesubstrate.
 13. A light detection element having the optical filteraccording to claim
 1. 14. An image pick-up element having the lightdetection element according to claim
 13. 15. A camera having the imagepick-up element according to claim
 14. 16. A spectral element having theoptical filter according to claim
 1. 17. The optical filter according toclaim 1, wherein the apertures are arranged in grid-like pattern. 18.The optical filter according to claim 1, wherein a wave-length of thelocalized plasmon is based on the size of the apertures.